Positive commandment · Mitzvat Aseh #60 of 613
For all animals to be sacrificed after their eighth day [of life]
להיות כל קרבנות בהמה מיום שמיני והלאה
The verse · Vayikra 22:27
שׁוֹר אוֹ־כֶשֶׂב אוֹ־עֵז כִּי יִוָּלֵד וְהָיָה שִׁבְעַת יָמִים תַּחַת אִמּוֹ וּמִיּוֹם הַשְּׁמִינִי וָהָלְאָה יֵרָצֶה לְקׇרְבַּן אִשֶּׁה לַה׳
“When a bullock, or a sheep, or a goat, is brought forth, then it shall be seven days under its dam; and from the eighth day and thenceforth it shall be accepted for an offering made by fire to the Lord.”
The Rambam’s words
For all animals to be sacrificed after their eighth day [of life], as [Leviticus 22:27] states: "And on the eighth day and afterwards...."
להיות כל קרבנות בהמה מיום שמיני והלאה, שנאמר: "מִיּוֹם הַשְּׁמִינִי וָהָלְאָה" (ויקרא כב, כז).
Who & when
“And [it] is practiced at the time of the [Temple] by the males of the priesthood — as the commandment of the sacrifice is upon them, and they will be sacrificed though them, and [so] it appears that they are warned (commanded) about the matter.”
Where in the Mishneh Torah
Hilchot Things Forbidden on the Altar הלכות איסורי המזבח